The present invention relates generally to digital communication systems, and more particularly to maximum likelihood sequence estimation for estimating a symbol sequence such as convolutional codes received via a communication channel having a time-varying impulse response.
Maximum likelihood sequence estimation is a well known technique for recovering a transmitted information sequence corrupted by intersymbol interference. The Viterbi algorithm is usually employed for implementation. Decoding a received symbol sequence using the Viterbi algorithm requires knowledge of the impulse response of the communication channel. If the communication channel is time-varying, an adaptation technique is necessary. As discussed in Digital Communications, John G. Proackis, McGraw-Hill, Second Edition, pages 624 et seq., a channel estimator is connected in parallel with a maximum likelihood sequence estimator (MLSE) to detect the impulse response of the channel with which the MLSE algorithm is adaptively controlled.
However, the prior art channel estimator is based on the first-order LMS (Least Mean Square) algorithm and is, therefore, incapable of adaptation to rapidly varying channel characteristics.